Magnetic materials are roughly classified into hard magnetic materials and soft magnetic materials. Hard magnetic materials are used for permanent magnets, digital devices, and the like.
Physical characteristics of hard magnetic materials include high magnetically anisotropic energy. Typical examples of hard magnetic materials include Nd—Fe—B, Nd—Dy—Fe—B, Sm—Co, Fe—Pt, Fe—Pd, Co—Pt, Tb—Fe—Co, Gd—Fe—Co, Co—Cr—Pt (each of which is an alloy material), Co/Pt, Co/Pd, Fe/Au (each of which is a nano-laminate material), and the like.
Typical examples of applications of permanent magnets include motors. There are various types of motors. For example, a permanent magnet formed of a hard magnetic material is used for a rotor that rotates within a space in which the polarity of a magnetic field varies with time.
A typical example of digital devices using a hard magnetic material is a hard disk drive. A hard disk drive includes a medium to store information by using the direction of the magnetization of a magnetic material and a head for storing information to the medium and reading information from the medium. The medium employs a hard magnetic material to hold information stored therein. In recent years, not only hard disk drives, but also magnetic random access memories have been studied actively as digital devices using a magnetic material. Some magnetic random access memories have been put into practice.
With regard to media of hard disk drives and storage elements of magnetic random access memories, recent development subjects have been shifted from in-plane magnetically anisotropic materials having an easy axis of magnetization within a plane of a substrate to perpendicular magnetically anisotropic materials having an easy axis of magnetization in a direction perpendicular to a substrate. This is because a perpendicular magnetically anisotropic material generally facilitates performance enhancement.
The structure of a magnetic random access memory will be described in greater detail below. A magnetic random access memory generally has a magnetic tunnel junction in which a magnetic layer, a non-magnetic layer, and a magnetic layer are stacked. One of the two magnetic layers has a variable direction of magnetization. Thus, the direction of magnetization varies when a magnetic field or an electric current is applied to the magnetic layer. The direction of magnetization of the other magnetic layer is perpetually fixed. As a matter of course, a hard magnetic material having relatively high magnetic anisotropy is required for the latter magnetic layer. For the former magnetic layer, a hard magnetic material having relatively high magnetic anisotropy is also required in order to hold information with stability. From the practical aspect, it is desirable to use a reactive ion etching method for patterning magnetic tunnel junctions in view of reduction in size, increase in capacity, reduction in cost, and the like.